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You can use generic routing encapsulation (GRE) as the carrier protocol for a variety of passenger protocols.
The following figure shows the IP tunnel components for a GRE tunnel. The original passenger protocol packet becomes the GRE payload and the device adds a GRE header to the packet. The device then adds the transport protocol header to the packet and transmits it.
Beginning Cisco NX-OS Release 9.3(1), you can configure both, segment routing MPLS and generic routing encapsulation (GRE) on a Cisco Nexus device. Both these technologies operate seamlessly. All MPLS traffic can be forwarded to the GRE tunnel after the MPLS tunnel termination. Similarly, you can forward all traffic from the GRE tunnel to the MPLS cloud after the GRE termination.
All PE routers can initiate, forward, or terminate the GRE traffic from or to another GRE cloud. Similarly, all tunnel transit or tunnel end nodes can configure MPLS tunnel encapsulation.
When both, the tunnel and segment routing is enabled on the Cisco Nexus 9000 switches, the following is the TTL behavior is for the respective flows:
Incoming IP traffic, egresses with GRE header, the TTL value in the GRE header is one less than the TTL value of the incoming IP packet.
Incoming IP traffic, egresses with MPLS header, the TTL value in the MPLS header is one less than the TTL value of the incoming IP packet.
Incoming GRE traffic, egresses with MPLS header, the TTL value in the MPLS header is default (255).
Incoming MPLS traffic, egresses with GRE header, the TTL value in the GRE header is default (255).
Segment routing MPLS and GRE have the following guidelines and limitations:
Ingress stats are not supported for tunnel packets.
Supported with either default or template-mpls-heavy mode only.
MPLS segment routing is not supported on the tunnel interfaces.
Due to a hardware limitation on the modular switches, the tunnel Tx traffic is not supported if the egress interface for the tunnel destination IP address is over the Cisco Nexus 9300-FX/FX2 platform switches.
Maximum four GRE tunnels are supported.
Beginning with Cisco NX-OS Release 9.3(3), you can configure both, segment routing MPLS and GRE on Cisco Nexus 9300-GX platform switches.
Tunnel Rx packet counters do not work when both segment routing MPLS and GRE coexist.
Cisco Nexus X98900CD-A and X9836DM-A line cards with 9808 and 9804 switches support SR MPLS default template only.
Support for Hierarchical ECMP to increase ECMP scale and accelerate convergence.
Hierarchical underlay ECMP imposes the same label stack on all connected next-hop members. Different label stacks for different NHs are not supported).
No support for statistics, hand-offs and consistency checker.
SR MPLS PHP node does not add an explicit NULL label while popping the last label.
In scenarios where Nexus 9804 switches interoperate with a node that adds an explicit NULL label during PHP, the TTL ignores the NULL label if a valid label is present after the NULL Label. The TTL from valid label is used for TTL decrement.
You can enable MPLS segment routing as long as mutually-exclusive MPLS features such as static MPLS are not enabled.
You must install and enable the MPLS feature set using the install feature-set mpls and feature-set mpls commands.
You must enable the tunneling feature using the feature tunnel command.
| Command or Action | Purpose | |||
|---|---|---|---|---|
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Step 1 |
configure terminal Example: |
Enters global configuration mode. |
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Step 2 |
[no] feature segment-routing Example: |
Enables the MPLS segment routing feature. The no form of this command disables the MPLS segment routing feature. |
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Step 3 |
(Optional) show running-config | inc 'feature segment-routing' Example: |
(Optional)
Displays the status of the MPLS segment routing feature. |
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Step 4 |
(Optional) copy running-config startup-config Example: |
(Optional)
Copies the running configuration to the startup configuration. |
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Step 5 |
configure terminal Example: |
Enters global configuration mode. |
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Step 6 |
feature tunnel Example: |
Allows the creation of a new tunnel interface. To disable the tunnel interface feature, use the no form of this command. |
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Step 7 |
switch(config)# interface tunnel number |
Enters a tunnel interface configuration mode. |
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Step 8 |
switch(config-if)# tunnel mode {gre ip } |
Sets this tunnel mode to GRE. The gre and ip keywords specify that GRE encapsulation over IP will be used. |
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Step 9 |
tunnel source {ip-address |interface-name} Example: |
Configures the source address for this IP tunnel. The source can be specified by IP address or logical interface name. |
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Step 10 |
tunnel destination {ip-address |host-name} Example: |
Configures the destination address for this IP tunnel. The destination can be specified by IP address or logical host name. |
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Step 11 |
tunnel use-vrf vrf-name Example: |
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Step 12 |
ipv6 address IPv6 address |
Configures the IPv6 address.
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Step 13 |
(Optional) switch(config-if)# show interface tunnel number |
(Optional)
Displays the tunnel interface statistics. |
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Step 14 |
switch(config-if)# mtu value |
Sets the maximum transmission unit (MTU) of IP packets sent on an interface. |
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Step 15 |
(Optional) switch(config-if)# copy running-config startup-config |
(Optional)
Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration. |
To display the segment routing MPLS and GRE configuration, perform one of the following tasks:
| Command | Purpose |
|---|---|
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show segment-routing mpls |
Displays segment routing mpls information |
To display the required details about the SRTE Explicit-path Endpoint Substitution configuration, perform one of the following tasks:
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Command |
Purpose |
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show srte policy |
Displays only the authorized policies.
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show srte policy [all] |
Displays the list of all policies available in the SR-TE.
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show srte policy [detail] |
Displays the detailed view of all the requested policies.
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show srte policy <name> |
Filters the SR-TE policy with the name and displays the list of all policies available with that name in the SR-TE.
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show srte policy color <color> endpoint <endpoint> |
Displays the SR-TE policy for the color and endpoint.
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show srte policy fh |
Displays the state of the existing first hop and policy endpoints. |
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